Docker execution environment in Selenium Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Selenium test inside Docker container

Consider this Selenium Java test code running inside a Docker container with ChromeDriver. What will be the output printed to the console?

Selenium Java

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;

public class DockerSeleniumTest {
    public static void main(String[] args) {
        System.setProperty("webdriver.chrome.driver", "/usr/bin/chromedriver");
        WebDriver driver = new ChromeDriver();
        driver.get("https://example.com");
        String title = driver.getTitle();
        System.out.println(title);
        driver.quit();
    }
}

A Example Domain

B NoSuchSessionException

C SessionNotCreatedException

D WebDriverException: unknown error: cannot find Chrome binary

💡 Hint

Think about what the test does after opening the page inside Docker with ChromeDriver.

The test opens https://example.com inside the Docker container using ChromeDriver and prints the page title. Since the container has Chrome and ChromeDriver properly installed, the output is the page title 'Example Domain'.

❓ assertion

intermediate

Correct assertion for page title in Docker Selenium test

Which assertion correctly verifies the page title is 'Welcome' in a Selenium Java test running inside Docker?

Selenium Java

String actualTitle = driver.getTitle();

A assertFalse(actualTitle.equals("Welcome"));

B assertNull(actualTitle);

C assertTrue(actualTitle == "Welcome");

D assertEquals("Welcome", actualTitle);

💡 Hint

Use the correct JUnit assertion method for string equality.

assertEquals("Welcome", actualTitle) correctly compares the expected string with the actual title. Using '==' compares references, which is incorrect for strings.

❓ locator

advanced

Best locator strategy for dynamic element in Docker Selenium test

Inside a Docker Selenium test, which locator is best to find a button with changing id but fixed visible text 'Submit'?

A By.xpath("//button[text()='Submit']")

B By.cssSelector("button#submitBtn")

C By.id("submitBtn")

D By.name("submit")

💡 Hint

Choose a locator that relies on stable visible text rather than dynamic attributes.

Using XPath with the button's visible text is reliable when the id changes dynamically. Locators by id or name may fail if those attributes change.

🔧 Debug

advanced

Identify cause of WebDriver timeout in Docker Selenium test

A Selenium test running inside Docker fails with a timeout waiting for element located by By.id("login"). What is the most likely cause?

A The test code uses incorrect syntax for locating elements

B The Docker container lacks network access to load the page

C The element with id 'login' does not exist on the page loaded inside Docker

D The ChromeDriver version is incompatible with Chrome browser inside Docker

💡 Hint

Consider what happens if the element is not present on the page.

Timeout waiting for element usually means the element is not present or visible. Network or version issues cause different errors. Syntax errors cause compile or runtime exceptions.

❓ framework

expert

Best practice for parallel Selenium tests in Docker environment

Which approach is best to run multiple Selenium tests in parallel inside Docker containers to avoid session conflicts?

A Run all tests in a single Docker container sharing one ChromeDriver instance

B Use a Selenium Grid with multiple Docker containers each running a separate browser instance

C Run tests sequentially inside one Docker container to avoid conflicts

D Use a single WebDriver instance shared across all test threads inside Docker

💡 Hint

Think about isolation and scalability for parallel tests.

Selenium Grid with multiple containers allows isolated browser sessions for parallel tests. Sharing one instance causes conflicts. Sequential runs do not achieve parallelism.

Practice

1. What is the main benefit of using Docker for Selenium test execution?

easy

A. It provides a clean, consistent browser environment for tests.

B. It automatically writes test scripts for you.

C. It replaces the need for assertions in tests.

D. It runs tests only on local machines.

Solution

1. Step 1: Understand Docker's role in Selenium testing

   Docker creates isolated containers that have clean browser setups, avoiding environment conflicts.

2. Step 2: Compare options

   Only It provides a clean, consistent browser environment for tests. correctly states Docker provides a clean, consistent browser environment. Other options describe unrelated or incorrect features.

3. Final Answer:

   It provides a clean, consistent browser environment for tests. -> Option A

4. Quick Check:

   Docker environment = clean browser setup [OK]

Hint: Docker isolates browsers for consistent tests [OK]

Common Mistakes:

• Thinking Docker writes test scripts automatically
• Believing Docker removes need for assertions
• Assuming Docker only runs tests locally

2. Which Java class is used to connect Selenium tests to a browser running inside a Docker container?

easy

A. DockerClient

B. ChromeDriver

C. FirefoxDriver

D. RemoteWebDriver

Solution

1. Step 1: Identify the correct Selenium class for remote connections

   RemoteWebDriver is designed to connect to browsers running on remote servers or containers like Docker.

2. Step 2: Eliminate other options

   ChromeDriver and FirefoxDriver are for local browsers. DockerClient is unrelated to Selenium.

3. Final Answer:

   RemoteWebDriver -> Option D

4. Quick Check:

   RemoteWebDriver connects to remote browsers [OK]

Hint: Use RemoteWebDriver for Docker browser connections [OK]

Common Mistakes:

• Using ChromeDriver for Docker browsers
• Confusing DockerClient with Selenium classes
• Using FirefoxDriver for remote Docker sessions

3. Given this Java code snippet connecting to a Docker Selenium container:

URL url = new URL("http://localhost:4444/wd/hub");
ChromeOptions options = new ChromeOptions();
RemoteWebDriver driver = new RemoteWebDriver(url, options);
driver.get("https://example.com");
String title = driver.getTitle();
System.out.println(title);

What will be printed if the page title is "Example Domain"?

medium

A. http://localhost:4444/wd/hub

B. Example Domain

C. ChromeOptions

D. RemoteWebDriver

Solution

1. Step 1: Understand the code flow

   The code opens the URL "https://example.com" in a remote Chrome browser inside Docker and retrieves the page title.

2. Step 2: Identify the output of getTitle()

   The getTitle() method returns the page title, which is "Example Domain" for that URL.

3. Final Answer:

   Example Domain -> Option B

4. Quick Check:

   driver.getTitle() = "Example Domain" [OK]

Hint: getTitle() returns the page title string [OK]

Common Mistakes:

• Printing the URL instead of the title
• Confusing options object with title
• Printing class names instead of values

4. You run Selenium tests inside Docker but get a connection refused error at "http://localhost:4444/wd/hub". What is the most likely cause?

medium

A. The URL "http://localhost:4444/wd/hub" is misspelled.

B. The test code uses ChromeDriver instead of RemoteWebDriver.

C. Docker container is not running or Selenium Grid is not started.

D. The test script lacks assertions.

Solution

1. Step 1: Analyze connection refused error

   This error means the test code cannot reach the Selenium server at the given URL.

2. Step 2: Identify common Docker causes

   If the Docker container running Selenium Grid is stopped or not started, the connection will fail.

3. Final Answer:

   Docker container is not running or Selenium Grid is not started. -> Option C

4. Quick Check:

   Connection refused = server not running [OK]

Hint: Check Docker container status if connection refused [OK]

Common Mistakes:

• Blaming ChromeDriver usage for connection errors
• Ignoring container running status
• Assuming URL typos without verification

5. You want to run parallel Selenium tests in Docker using RemoteWebDriver with ChromeOptions. Which approach best ensures tests run independently without interference?

hard

A. Start multiple Docker containers each running a Selenium node and connect each test to a different container URL.

B. Run all tests sequentially in a single Docker container using the same RemoteWebDriver instance.

C. Use ChromeDriver locally and ignore Docker for parallel tests.

D. Run tests in one container but share the same RemoteWebDriver instance across threads.

Solution

1. Step 1: Understand parallel test isolation needs

   Parallel tests require isolated browser sessions to avoid state conflicts.

2. Step 2: Evaluate Docker container usage

   Starting multiple containers with separate Selenium nodes allows independent browser sessions for each test.

3. Step 3: Eliminate unsafe options

   Sharing RemoteWebDriver instances or running sequentially defeats parallelism. Using local ChromeDriver ignores Docker benefits.

4. Final Answer:

   Start multiple Docker containers each running a Selenium node and connect each test to a different container URL. -> Option A

5. Quick Check:

   Parallel tests need isolated containers [OK]

Hint: Use separate containers for parallel Selenium tests [OK]

Common Mistakes:

• Sharing RemoteWebDriver instance across threads
• Running tests sequentially to avoid conflicts
• Ignoring Docker and using local drivers